In the machine tool, the spindle device used in such machine tool is of a structure, in which a main shaft front end side is supported relative to a spindle housing by a rolling bearing unit of a radial type capable of supporting an axial load and inner and outer ring spacers are provided proximate to the rolling bearing unit. At the front end side of the main shaft, a jig used in the machine tool is detachably mounted through a collet chuck. The jig clamped by the collet chuck is unclamped by a chuck drawing bar. The Patent Document 1 listed below discloses a technique associated with clamping and unclamping of the machine tool by means of the chuck drawing bar and the collet chuck operatively associated therewith.
With respect to this type of the spindle device for the machine tool, the applicant of the present invention has mass-produced and marketed such a product that in the event that the unclamping force relative to the clamping force of the collet chuck exceeds a permitted axial load of the rolling bearing unit, a portion of the axial load, which has exceeded the permitted axial load, can be supported by spacers provided in proximate to the rolling bearing unit. By employing this structure, even in the spindle device of the machine tool, in which a large unclamping force acts, there is no need to use either a bearing unit of a large size or a bearing unit specially designed, whichever for supporting the large unclamping force, and the spindle device can be manufactured compact in size and at a reduced cost.
[Patent Document 1] JP Patent No. 3415211
FIG. 5 is a sectional representation showing a rolling bearing assembly, used in the spindle device, and its periphery according to the product of the applicant mentioned above. A pair of rolling bearing units 3 and 3 are mounted on an outer periphery of a main shaft 1 and juxtaposed in a direction axially thereof, and an inner ring spacer 4 and an outer ring spacer 5 are interposed between the rolling bearing units 3 and 3. The inner ring spacer 4 and the outer ring spacer 5 are provided with respective load bearing portions 4b and 5c, which are so arranged as to confront with each other in the axial direction with a gap δ intervening therebetween. When the axial load exceeding the permitted axial load acts on the rolling bearing units 3 by the effect of an unclamping force P or the like acting on the main shaft 1, inner and outer rings 3a and 3b of each of the rolling bearing units 3 displace axially relative to each other. If such displacement takes place in a quantity exceeding a predetermined value, the load bearing portion 4b in the inner ring spacer 4 and the load bearing portion 5c in the outer ring spacer 5 are brought into contact with each other so that the axial load exceeding the permitted axial load can be received by the load bearing portions 4b and 5c. For this reason, the rolling bearing units 3 suffice if the permitted axial load can be secured during processing.
It has, however, been found that since the axial displacement of the inner and outer rings 3a and 3b of each of the rolling elements 3 depends considerably on the processing accuracy or the like of a machine tool, if the spindle device is so constructed as hereinabove described, it is necessary to severely control the dimensional accuracy of the gap δ delimited between the inner and outer ring spacers 4 and 5, which regulates the above described displacement. In order to secure the gap δ of a high dimensional accuracy, processing has to be performed while the dimensions of each of the inner ring spacer 4 and the outer ring spacer 5 is measured occasionally, followed by an adjustment process to be performed to secure the required gap δ. For this reason, it is difficult to manufacture the inner ring spacer 4 and the outer ring spacer 5, involving a problem associated with requirement of a substantial amount of labor in processing and a substantial cost.